A projection arch surface (or “projection arch” for short) describes the arch surface that is clearly mapped in the resultant X-ray image of a panorama X-ray apparatus. In other words, the X-ray image displays the patient's tissue that is located in the projection arch surface when the X-ray is taken.
During the taking of a dental panorama X-ray image, the patient's head is located between an X-ray source and an X-ray detector which circumscribe the patient's head in a curved path. The X-ray source and the detector unit are for this purpose generally rotated, at a uniform distance from one another in a horizontal plane, about a vertical axis of rotation extended, in turn, along a specific path during the rotation process. The ideal course of this path results from the geometry of a patient's dental arch. In the region of the front teeth, the dental arch displays a marked curvature, whereas in the region of the central or rear teeth the curvature is less pronounced.
As the X-ray source and the detector unit move around the patient's head, a large number of individual photographs are taken which are combined by a suitable computing unit to form an overall image.
There is to be allocated to each individual photograph a narrow, planar projection region in which the patient's tissue through which the X-rays pass is clearly mapped. To put it simply, relatively narrow horizontal regions of individual images are therefore combined in each case to form the panorama X-ray image, the axis of rotation of the X-ray source and detector unit moving in accordance with a standard jaw shape.
The projection arch surface of the panorama X-ray apparatus is obtained from the correspondingly combined horizontal regions of the projection planes of the respective individual images or image strips. Overall, the projection arch surface of the panorama X-ray apparatus results from the path in which the X-ray source and the detector unit circumscribe the patient's head and which generally has to be set manually or on the basis of data from a database.
There are also further recording settings of the dental panorama X-ray apparatus to be carried out including, inter alia, the tube voltage, the exposure time, the beam current and the beam cross-section as parameters for the X-ray source.
In order for the resultant X-ray image also to display the desired regions of the patient, the patient has to be positioned in a specific set relative position in the panorama X-ray apparatus, in which position the largest possible area of the patient's dental arch coincides with the projection arch surface.
The positioning of the patient, like the adjusting of the panorama X-ray apparatus is conventionally carried out manually. The panorama X-ray apparatus accordingly has a bite-on-holder that the patient bites into. The patient is then positioned, usually using an optical device, in such a way that the plane between his upper and his lower jaw extends horizontally. The projection arch is then manually adjusted in accordance with the patient's dental arch visually detected by the operator, and the patient is brought between the X-ray source and the X-ray detector vertically and horizontally into a position such that the projection arch surface of the panorama X-ray apparatus extends as precisely as possible through the roots of the patient's teeth.
The above-mentioned manual adjustments are firstly very time-consuming and secondly highly prone to error. More than half of panorama X-ray photographs taken are inadequate, as the position arch surface does not extend as desired through the roots of the teeth which are therefore mapped unclearly.
A reduction in the sources of error was achieved by a device of the type mentioned at the outset such as is described in DE 38 08 009 C2. In this device, the position of the patient's incisors and/or canines is established using various types of sensor. The relative positions of the detected teeth with respect to one another are used as the basis for determining the set relative position of the patient in the panorama X-ray apparatus. In order to determine the patient's dental arch, use is made of a database in which physiological patient data is stored. From this database there is allocated to the patient to be examined a dental arch geometry corresponding as closely as possible to the shape of the dental arch established from the positions of the incisors and/or canines of the patient to be examined.
As merely the patient's incisors and/or canines are detected for determining the patient's dental arch, there can be allocated to the patient from the database a dental arch geometry which differs from his actual dental arch shape and, in particular, takes in the central and rear jaw regions a different course from that actually found in the patient.
It can therefore occur that the X-ray image taken does not represent the region of the patient's molars and grinders with sufficient clarity and is unsuitable for subsequent diagnosis by the dentist performing the treatment.
The present invention is directed to addressing these and other matters.